Neighbor insentive pixel deletion method for printing high resolution image

ABSTRACT

To print an image having a 600 dpi resolution along the horizontal axis with an ink jet printer that has a 300 dpi resolution, the method maintains the edges of the pixel image and uses a checkerboard pattern for all interior pixels other than the second pixel and the second last pixel of each pixel row. Therefore, the method is neighbor insensitive by using a checkerboard pattern or mask to determine the state of the interior pixels. After turning OFF the respective pixels, ink drops are fired from the ink jet printer at areas corresponding to the remaining ON pixels. This visibly reproduces the image at the 600 dpi resolution.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to ink jet printers and more particularly, to amethod of altering a high resolution pixel image to produce a pixelimage that is capable of being visibly reproduced by an ink jet printerhaving a lower resolution.

2. Description of Related Art

An ink jet printhead selectively ejects droplets of ink from a pluralityof drop ejectors to create a desired image on an image receiving medium,such as paper. The printhead typically comprises an array of dropejectors that convey ink to the image receiving medium. In acarriage-type ink jet printhead, the printhead moves back and forthrelative to the image receiving medium to print the images in swaths.

The ink jet printhead typically comprises a plurality of inkpassageways, such as capillary channels. Each channel has a nozzle endand is connected to an ink supply manifold. Ink from the manifold isretained within each channel until, in response to an appropriate signalapplied to a resistive heating element in that channel, the ink in aportion of the channel adjacent to the heating element is rapidly heatedand vaporized. Rapid vaporization of some of the ink from the channelcreates a bubble that causes a quantity of ink (i.e., an ink droplet) tobe ejected through the nozzle to the image receiving medium. U.S. Pat.No. 4,774,530 to Hawkins, the disclosure of which is incorporated hereinby reference, shows a general configuration of a typical ink jetprinter.

Many commercially available ink jet printers have a 300 dots per inch(dpi) resolution along the horizontal axis (also called the raster scanaxis). Ink jet printers also typically have a 300 dpi verticalresolution so as to form round individual ink droplets that form thepixel images on the paper. These ink jet printers are therefore designedto print 300 by 300 dpi output images. However, increased resolution isdesired to print higher resolution images. For example, it may bedesirable to print a 300 by 600 dpi pixel image using a 300 dpi ink jetprinthead. The effective resolution of an ink jet printer can beincreased along the horizontal axis (or raster scan axis) by energizingor firing the ink jets at a higher rate. This requires redesigning theink jet firing head and therefore results in a substantially highercost. It is also possible to increase the effective resolution along theraster scan axis by slowing down the rate at which the printhead movesacross the paper. However, this may cause excessive ink to be depositedon the paper due to excessive ink drop overlap.

U.S. Pat. No. 5,270,728 to Lund et al., the disclosure of which isincorporated herein by reference, discloses a method for multiplying thespeed resolution of a raster scanning device such as an ink jet printer.A 300×600 dpi pixel image is mapped to a corresponding, non-overlappingphysical dot image and the ink jets are fired in response to the dotimage to direct ink droplets onto the paper at 600 dpi resolution gridtiming to effectively double the horizontal resolution of the pixelimage. This is done without increasing the firing rate of the printhead.

U.S. Pat. No. 5,270,728 describes thinning the pixel image before it isprinted by selectively turning off pixels within the pixel image byreferencing the edges of the pixel image. More specifically, U.S. Pat.No. 5,270,728 describes a method of maintaining selected ones of thepixels in an ON state based on ON pixel adjacency and edge proximitycriteria.

FIG. 1 shows a pixel image 200 that is desired to be printed by the inkjet printer. In FIG. 1, each pixel is represented by a correspondingshaded rectangle having a 300 dpi vertical and 600 dpi horizontalresolution. For ease of illustration, each row is consecutively numbered10, 11, 12, 13, 14, 15, 16 and 17. Each of the columns of the pixelimage 200 is labeled with a corresponding letter A, B, C, D, E, F, G andH. The columns are labeled across the raster scanning (or horizontal)direction X. The rows are labeled in the slow scan (or vertical)direction Y. As an example, the pixel in the upper left hand corner ofthe pixel image 200 is labeled as pixel 10C. Similarly, the pixel in thebottom right hand corner is labeled as pixel 17G.

FIG. 2 shows an altered pixel image 210 according to the methoddescribed in U.S. Pat. No. 5,270,728. As can be seen by FIG. 2, selectedpixels are turned OFF so that no two adjacent pixels of the originalpixel image 200 are maintained ON for a given row. Furthermore, the lefthand edge of the original pixel image 200 is always maintained in an ONstate. Furthermore, the second to last pixel in each of the respectiverows is always maintained ON while turning OFF the last pixel in eachrow. For example, pixels 10C and 10G are maintained ON while pixel 10His turned OFF. Further, interior ON pixels are maintained ON based ontheir proximity to the edges of the image. Thus, the method described inU.S. Pat. No. 5,270,728 is concerned with determining the proximity (orcloseness) of a pixel to an edge of a pixel row.

As can be seen by FIG. 2, the method of U.S. Pat. No. 5,270,728 mayresult in a vertical column of pixels being turned OFF. For example,this can be seen by pixels 12E, 13E, 14E, 15E, 16E and 17E. The OFFpixels in column E can lead to visual problems (such as visual texture)when the altered pixel image 210 is mapped and printed according to themethod described by U.S. Pat. No. 5,270,728.

FIG. 3 shows a mapping process described in U.S. Pat. No. 5,270,728whereby each pixel that was maintained ON during the altering step(i.e., following FIG. 2) is subsequently mapped with an OFF pixel to itsimmediate right. For example, dot 60 corresponds to ON pixel 10C and OFFpixel 10D. The resulting dot structure represents the 300 dpi resolutionof the printhead. Thus, the altered pixel image 210 described in FIG. 2is mapped so that each ON pixel is mapped to a dot 60 with acorresponding OFF pixel to its immediate right. The dot structure ofFIG. 3 is then raster scanned by the ink jet printer to print the pixelimage.

U.S. Pat. No. 5,270,728 suffers from several problems that effect thevisual quality of the resulting image on the copy sheet. For example,when all the pixels in a respective column are turned OFF such as incolumn E for rows 12-17, visual texture problems may result on the copysheet. These problems occur in part because the method described in U.S.Pat. No. 5,270,728 relies on edge proximity criteria to determine howclose respective pixels are to an edge. While it may sometimes bedesirable to be sensitive to the edge, this results in a complicatedmethod that requires the numerous pixels within each row to be examinedbefore each pixel is appropriately maintained ON or turned OFF.

SUMMARY OF THE INVENTION

To solve these and other problems, this invention provides a method ofincreasing the effective resolution of a printhead without the need foredge proximity criteria. This invention therefore provides a muchsimpler method than that described in U.S. Pat. No. 5,270,728 andaccordingly results in an easier to implement method.

This invention also provides a method of printing an image having adefined resolution along a given axis using an ink jet printer that hasa predetermined resolution less than the defined resolution of theimage. The image is received having the defined resolution along thegiven axis. The second pixel and the second last pixel of eachcontiguous row of ON pixels are subsequently turned OFF. Interior pixelsof the image are altered based on a checkerboard pattern or mask. Afterthe altering step, the remaining pixels of the image are raster scannedwith the ink jet printer to visibly reproduce the image at the definedresolution.

Other objects, advantages and salient features of the invention willbecome apparent from the following detailed description taken inconjunction with the annexed drawings, which disclose preferredembodiments of the invention.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described with reference to the following drawingsin which like reference numerals refer to like elements and wherein:

FIG. 1 is a pixel image desired to be printed;

FIG. 2 is an altered pixel image according to a method described in U.S.Pat. No. 5,270,728;

FIG. 3 is a mapped dot structure according to a method described in U.S.Pat. No. 5,270,728;

FIG. 4 is a schematic view of an ink jet printing system;

FIG. 5 is a flowchart describing an embodiment of the present invention;

FIG. 6 is an altered pixel image according to an embodiment of thepresent invention;

FIG. 7 is an embodiment of mapped pixels corresponding to ink drops; and

FIG. 8 shows how the address parity may be checked for a swath of pixeldata.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 4 shows a carriage-type ink jet printing device 20. A linear arrayof droplet-producing channels is housed in the printhead 22 of areciprocal carriage assembly 24. Ink droplets 26 are propelled to areceiving medium 28 that is stepped by a motor 30 a preselected distancein the direction of arrow 32 each time the printhead 22 traverses acrossthe receiving medium 28 in the direction indicated by arrow 32. Thereceiving medium 28,such as paper, can be stored on the supply roll 34and stacked onto a take-up roll 36 by a stepper motor 30 or other meanswell known in the art.

The printhead 22 is fixedly mounted on the support base 38, which isadapted for reciprocal movement using any well known means such as thetwoparallel guide rails 40. The reciprocal movement of the printhead 22may beachieved by a cable 42 and a pair of rotatable pulleys 44, one ofwhich is powered by a reversible motor 46. The printhead 22 is generallymoved across the receiving medium 28 perpendicularly to the directionthe receiving medium 28 is moved by the motor 30. Of course, otherstructures for reciprocating the carriage assembly 24 are also knownwithin the art.

The printhead 22 may also be connected to a controller 50 that will beusedwith embodiments of the present invention to receive a pixel imageat a resolution greater than that of the printhead 22. The controller 50may include hardware or software to carry out the present invention aswill bedescribed below. Further, it is understood that the abovedescription of the ink jet printing device 20 is merely illustrative andis not limiting.That is, other structures are also within the scope ofthis invention.

FIG. 5 shows a flowchart describing an embodiment of the presentinvention that will be described with respect to FIGS. 6 and 7 for asingle pixel row. However, as will be described below, it is understoodthat operation may occur for an entire swath of pixel data wherebyseveral pixels (normally the height of the printhead) will besimultaneously considered in parallel.

Initially, a pixel image 200 is received in the controller 50 in step100. The image may be created from a scanning device, a ROS, a computeror other similar devices. For illustration purposes, the image that isreceived is the original pixel image 200 shown in FIG. 1. In step 102, aparity indicator N is set to one at the start of an even numbered row asthe pixel row is being examined in a left-to-right manner on an evenrow, and set to zero at the start of an odd numbered row. For example,when examining the original pixel image 200 from FIG. 1, the parityindicator Nwill be set to one upon the controller 50 examining pixel10A, but zero on examining pixel 11A. While this embodiment will bedescribed with respect to step 102 as including a parity indicator, itwill be appreciated that this step may inherently be a part of othersteps such as, for example, step 112. In step 103, the state of thecurrent pixel is tested. If it is ON, operation continues to step 104,otherwise operation continues to step120.

In step 104, a determination is made whether the current pixel beingexamined is a left edge pixel (i.e., whether the previous pixel was OFFinthe input and the current pixel is ON). If so, the current pixel ismaintained ON in step 114. For example, FIG. 6 shows pixel 10C beingmaintained ON. Operation continues to step 120 where the parityindicator N is toggled (by subtracting it from one). The next pixel tobe consideredis pixel 10D. In step 104, it is determined that thecurrent pixel 10D is not a left edge pixel and operation proceeds tostep 106. Because pixel 10D is the second pixel from the left edge pixel(i.e., the previous pixelwas a left edge pixel), the current pixel 10Dis turned OFF in step 116. Again, the parity indicator N is toggled instep 120 and the next pixel is10E. Decision steps 103, 104 and 106 aresubsequently repeated. In steps 108 and 110 it is determined that thecurrent pixel 10E is not the right edge pixel or the pixel before theright edge pixel of the row. Operation continues to step 112. In step112, a pixel is maintained ON if the parityindicator is one, orequivalent, whether the pixel location is one of the black squares of anunderlying checkerboard pattern or mask. Otherwise, the current pixel10E is turned off.

The parity indicator follows a checkerboard pattern (or mask) 300 suchas that shown in FIG. 8. The checkerboard pattern 300 shown in FIG. 8may be stored in either software or may be performed by hardware logic.FIG. 8 shows a checkerboard pattern 300 eight rows high and eight pixelswide. This illustrates part of the grid area that the pixel image 200may be formed within. It is understood that this checkerboard pattern300 is in no way limited to the size described above. In one embodiment,the pixel 10E is compared in step 112 with the state of the respectiveposition 10E on the checkerboard pattern 300 showing a logical one. Byperforming an AND operation between the parity indicator at the pixel10E (a logical one) and the ON state of pixel 10E (a logical one) it isdetermined that the interior pixel 10E matches the parity indicator.Accordingly, the current pixel 10E is maintained ON in step 112. Othermethods exist in software. For example, in software a byte at a time ofthe interior portion of the image can be logically ANDed with thehexadecimal value O×55 or O×AA for odd and even rows, respectively.

The parity indicator N is again toggled in step 120, and the next pixelto examine is 10F. Operation continues through steps 103, 104, 106, 108,110 and arrives at step 112 where again the parity indicator is ANDedwith thestate of pixel 10F. Interior pixel 10F (having an ON state inthe original pixel image 200) is compared with the parity indicator withthe state shown in FIG. 8 (a logical zero). By performing an ANDoperation between the ON state and the parity indicator, it isdetermined that pixel 10F will be turned OFF in the altered image 220 ofFIG. 6.

Again, the parity indicator N is toggled in step 120 and proceedsthrough steps 103, 104 and 106. In step 108, pixel 10G is determined tobe the second last pixel in row 10. Accordingly, pixel 106 is turned OFFin step 116. Finally, the parity indicator N is toggled in step 120, andthe controller proceeds to examine pixel 10H and repeats steps 103, 104,106 and 108. In step 110, pixel 10H is determined to be the right edgepixel. This pixel is maintained ON in step 118. Operation continues forthe subsequent pixel rows in a similar manner. However, as explainedabove, operation may occur for a plurality of rows simultaneously (i.e.,in parallel).

The embodiment described above with respect to FIG. 5 maintains the leftedge pixel and the right edge pixel ON for each respective contiguousrow of ON pixels. Furthermore, the second pixel from the left edge andthe second last pixel in a row are turned OFF. The remaining interiorpixels within the original pixel image 200 are appropriately alteredusing a checkerboard pattern or mask.

FIG. 7 shows each remaining ON pixel in the altered pixel image 220being mapped to a dot 60 corresponding to a 300 dpi ink droplet so as toform a mapped image 230. For example, each ON pixel in the altered image220 is mapped with an OFF pixel to its immediate right. The alteredpixel image 220 within the controller 50 is raster scanned by theprinthead 22 to produce the dots 60 on the receiving medium 28 so as tovisibly reproduce the pixel image 200 at the printhead's resolution. Themapping can result in "enlarging" the image as the last dots in each ofthe rows extend beyond the ends of the pixel rows of the original pixelimage 200.

Accordingly, the above-described method does not require as complicatedan algorithm as that described in U.S. Pat. No. 5,270,728. Furthermore,the present invention avoids problems of the prior art caused bysimilarly altered different rows such as rows 13-16 of FIG. 3 that maysubsequently result in visual textural problems. Furthermore, thealgorithm of U.S. Pat. No. 5,270,728 is very sensitive to both the leftand right hand edgesand accordingly requires proximity to be a majorcriteria for determining whether pixels should be maintained ON orturned OFF. However, the presentinvention uses a checkerboard pattern ormask such as that of FIG. 8 to easily turn pixels OFF or maintain pixelsON within the interior regions of the pixel image 200.

The above described method is preferably performed in the controller 50using software on a computer where the bits of each scan line (i.e.,pixelrow) are considered serially. That is, each row is examined in aleft-to-right manner. While the invention may be in practice in softwareas described above, it may also be incorporated into hardware logic.

An ink jet printer with a reciprocating printhead 22 normally has somenumber (typically 48 or more) of jets or channels that are configured inan orientation perpendicular to the direction of travel of the printhead22. All of these jets or channels can fire essentially simultaneously toproduce a vertical column of pixels at each location. In this way, asthe printhead 22 traverses the width of the receiving medium 28, anentire swath of pixels is printed. Each swath is a set of pixels thewidth of thereceiving medium 28 and the height of the printhead 22.Because firing of the jets or channels must be precisely timed, adedicated memory buffer (also referred to as a swath buffer) is usedwithin the controller 50 to store the image data for an entire swathprior to the traverse of the printhead 22 across the receiving medium28. The invention may operate on the data in the swath buffer prior toits transmission to the printhead 22.

Hardware logic may be provided to always maintain the left edge pixel ina contiguous row of ON pixels in an ON state and turn OFF the secondpixel for each such row. Additionally, the second last pixel is turnedOFF whilethe last pixel of each row is maintained ON. This can beaccomplished by looking forward to the next pixel to see of the nextpixel is OFF in the original pixel image 200 (i.e, then the currentpixel is a right edge pixel). To obtain the checkerboard pattern or maskshown in FIG. 8, a counter (or flip-flop) may be provided for each ofthe respective rows to alternate between logic 1's and logic 0's. Forexample, as the printhead 22 traverses across the receiving medium 28,the pixel rows 10, 12, 14 and16 will alternatively switch between 1'sand 0's starting with a logic 1. The pixel rows 11, 13, 15 and 17 willalternate between 0's and 1's starting with a logic 0. Therefore,whenever a consecutive block of ON pixels is encountered, the firstpixel will be maintained ON, the second pixel will be turned OFF, andany subsequent pixels within the pixel row (other than the last twopixels) will follow the respective counter state as determined by theposition of the printhead. This will therefore produce the checkerboardpattern within the interior region of the original pixel image 200.

While the invention has been described in relation to preferredembodiments, many modifications and variations are apparent from thedescription of the invention. All such modifications and variations areintended to be within the scope of the present invention as defined intheappended claims.

What is claimed is:
 1. A method of printing an image with at least anink jet printer, the image having a defined resolution along a givenaxis, the ink jet printer having a resolution less than the definedresolution of the image, the image comprising a plurality of pixel rowseach extending from a respective first edge pixel to a respective secondedge pixel, the method comprising the steps of:maintaining the firstedge pixel as an ON pixel and the second edge pixel as an ON pixel;turning OFF selected interior pixels within the image based on acheckerboard pattern; and raster scanning remaining pixels of the imagewith the ink jet printer to visibly reproduce the image at the definedresolution.
 2. A method of printing an image with at least an ink jetprinter, the image having a defined resolution along a given axis, theink jet printer having a resolution along the given axis less than thedefined resolution of the image, the image comprising a plurality ofpixel rows each extending from a respective first edge pixel to arespective second edge pixel, the method comprising the stepsof:receiving the image having the defined resolution along the givenaxis; maintaining said first edge pixel as an ON pixel and said secondedge pixel as an ON pixel; for each pixel row, turning OFF each pixelcorresponding to a second pixel of said each pixel row and a second tolast pixel of said each pixel row, the second to last pixel being apixel within the image just prior to said second edge pixel for anygiven row; altering interior pixels of the image between the deletedpixels of said each row based on a checkerboard pattern; and after thealtering step, raster scanning remaining pixels of the image with theink jet printer to visibly reproduce the image at the definedresolution.
 3. The method of claim 2, wherein the checkerboard patternis stored in a memory of a controller, the altering step being performedin the controller.
 4. The method of claim 2, wherein the altering stepuses hardware logic to simulate the checkerboard pattern.
 5. The methodof claim 2, wherein the defined resolution along the given axis isapproximately 600 dots per inch and the resolution of the ink jetprinter along the given axis is approximately 300 dots per inch.
 6. Themethod of claim 2, wherein the altering step results in each row havingno adjacent ON pixels within the image.
 7. The method of claim 2,wherein the altering step results in each row having no interior ONpixels within the image vertically adjacent to interior ON pixels withinthe image of an adjacent pixel row.
 8. The method of claim 2, whereinthe altering step occurs substantially simultaneously for said pluralityof pixel rows.
 9. A method of printing an image with at least an ink jetprinter, the image having a defined resolution along a given axis, theimage having a resolution along the given axis less than the definedresolution of the image, the image comprising a plurality of pixel rowseach extending along the given axis from a first edge pixel to a secondedge pixel, interior pixels being pixels between the first edge pixeland the second edge pixel of each pixel row, the method comprising thesteps of:receiving the image having the defined resolution along thegiven axis; maintaining the first edge pixel as an ON pixel and thesecond edge pixel as an ON pixel: turning OFF selected interior pixelswithin the image based on a checkerboard pattern to form an alteredinterior image; and firing ink drops from the ink jet printer at areascorresponding to remaining ON pixels of said altered interior image andthe first edge pixel and the second edge pixel, the fired ink dropsvisibly reproducing the image at the defined resolution along the givenaxis.
 10. The method of claim 9, wherein the altered interior image hasan absence of consecutive ON interior pixels for any pixel row and anabsence of adjacent ON interior pixels between adjacent pixel rows. 11.The method of claim 9, wherein the defined resolution along the givenaxis is approximately 600 dots per inch and the resolution of the inkjet printer along the given axis is approximately 300 dots per inch. 12.The method of claim 9, wherein the receiving step comprises receivingthe image in a memory.
 13. The method of claim 12, wherein the turningOFF step is performed in the memory.
 14. The method of claim 9, furthercomprising the step of, for each row, turning OFF pixels adjacent tosaid first edge pixel and said second edge pixel.
 15. The method ofclaim 9, further comprising the step of maintaining ON the first edgepixel and the second edge pixel for each pixel row.
 16. An ink jetprinting method for printing with at least an ink jet printer, the inkjet printer having a resolution along a raster scanning direction, themethod comprising the steps of:receiving a pixel image having a definedresolution in the raster scanning direction greater than the resolutionof the ink jet printer, the pixel image comprising a first pixel rowextending in the raster scanning direction from a first edge pixel to asecond edge pixel and a second pixel row extending in the rasterscanning direction from a third edge pixel to a fourth edge pixel;altering the first pixel row by maintaining ON selected pixels andturning OFF selected pixels of the first pixel row, the altered firstpixel row having the first edge pixel and the second edge pixel beingmaintained as ON selected pixels; altering the second pixel row bymaintaining ON selected pixel and turning OFF selected pixels of thesecond pixel row, the altered second pixel row having no adjacent ONinterior pixels, the third edge pixel and the fourth edge pixel beingmaintained as ON selected pixels, the altered second pixel row having noON interior pixels vertically adjacent to ON interior pixels of thefirst pixel row; and raster scanning the altered first pixel row and thealtered second pixel row with the ink jet printer to visibly reproducethe image at the defined resolution.
 17. The method of claim 16, whereinthe first pixel row and second pixel row altering steps are performedusing a checkerboard pattern.
 18. The method of claim 16, furthercomprising the step of altering subsequent pixel rows by maintaining ONthe edge pixels as ON pixels, by maintaining ON selected interior pixelsand by turning OFF selected interior pixels of the subsequent pixelrows.
 19. The method of claim 18, wherein the ON selected interiorpixels are maintained ON based on a checkerboard pattern and the OFFselected interior pixels are turned OFF based on the checkerboardpattern.
 20. The method of claim 18, wherein each altered subsequentpixel row has no adjacent ON interior pixels and no ON interior pixelsvertically adjacent to ON interior pixels of an adjacent pixel row. 21.The method of claim 16, wherein the altering step includes the step ofselectively examining each pixel in the first pixel row, starting withthe first edge pixel to label each pixel as one of an ON pixel and anOFF pixel, the first edge pixel and the second edge pixel being labeledas ON pixels.
 22. The method of claim 16, wherein the altering stepincludes the step of selectively examining each pixel in the secondpixel row, starting with the third edge pixel to label each pixel as oneof an ON pixel and an OFF pixel, the third edge pixel and the fourthedge pixel being labeled as ON pixels.